Water-assisted oxidative redispersion of Cu particles through formation of Cu hydroxide at room temperature

Sintering of active metal species often happens during catalytic reactions, which requires redispersion in a reactive atmosphere at elevated temperatures to recover the activity. Herein, we report a simple method to redisperse sintered Cu catalysts via O2-H2O treatment at room temperature. In-situ spectroscopic characterizations reveal that H2O induces the formation of hydroxylated Cu species in humid O2, pushing surface diffusion of Cu atoms at room temperature. Further, surface OH groups formed on most hydroxylable support surfaces such as γ-Al2O3, SiO2, and CeO2 in the humid atmosphere help to pull the mobile Cu species and enhance Cu redispersion. Both pushing and pulling effects of gaseous H2O promote the structural transformation of Cu aggregates into highly dispersed Cu species at room temperature, which exhibit enhanced activity in reverse water gas shift and preferential oxidation of carbon monoxide reactions. These findings highlight the important role of H2O in the dynamic structure evolution of supported metal nanocatalysts and lay the foundation for the regeneration of sintered catalysts under mild conditions.

3. In line 255-256, the conclusion of "The Cu parficle size may be the decisive factor for the different catalyfic performance in the two reacfions" may be a bit overstated.This is because in this specific study, the oxidafion state of the Cu species is closely related to the parficle size.However, this may not always be true for other recipe of catalyst preparafions.In other words, the valence state of the Cu species may also contribute important roles in these reacfions.
Reviewer #3 (Remarks to the Author): Fan et al. report on the effect of wet pretreatments of supported Cu nanoparficles on metal dispersion.While some interesfing data are produced, I am not sure that the arficle meets the standards of Nature Communicafions.1.The introducfion does not summarize the current knowledge on metal redispersion.It is more focused on a few examples dealing with Cu. 2. The main oxide supports, referred to as AlOOH-500 and -900, are claimed to be -Al2O3 (page 4), but there is no evidence for that phase.3. Electron microscopy images in Figs. 1 and 4 show scale bars with no indicafion of the scale.In Fig. 1a, the supposed single Cu atoms are surrounded by circles that make any visualizafion impossible.4. In Fig. 1f, EXAFS data are reported.However, neither XANES curves nor EXAFS fifting is provided.The components at R=4-5 Å are not ascribed for the Cu foil. 5.Moreover, sfill for EXAFS, why only 2Cu/AlOOH-900 sample treated in O2-H2O is reported?One would at least expect the data for this sample before treatment.6.The XPS data in Fig. 2b are hardly visible, hidden by thick fifting curves.Moreover, why are the 2p1/2 components not shown?7. H-D exchange experiments: describe them and how they can be interpreted in terms of OH content, since nothing can be understood just from the claim "H-D exchange results show that Ar-H2O treatment increases the OH content…".8. Page 8, what is the "spontaneous monolayer dispersion phenomenon"?The way it is described, it looks more like coalescence than dispersion.9.By the way, the use of the "dispersion" term throughout the paper is often improper."Dispersion" refers to the fracfion of surface metal atoms in a nanoparficle.The process leading to single atoms or clusters from bigger enfifies (such as nanoparficles) is called "redispersion".10.Page 11, it is menfioned that XPS shows that hydroxylated Cu species "can" form on Si3N4, but H-D exchange indicates that few OH groups are present on Si3N4.However, it is concluded that surface hydroxylafion is key to Cu redispersion.How does it account for XPS results?11.Catalyfic data (Fig. 6) look minimalist.For example, I would expect several successive heafing/cooling cycles to assess the stability of the process.Would the two samples keep exhibifing different behaviors?12.The experimental data are only parfial.For instance, plenty of parameters such as reactor diameter, flow rates, catalyst weight, etc., are omifted.Isn't the conversion formula (page 17) valid for CO oxidafion?
13. Fig S8d : the flat background is surprising.14.Check "corporafion" (page 3) and "conducive" (page 13) terms.Overall, due to the lack of important informafion as well as the poor analysis and presentafion of the data, I do not recommend this paper for publicafion, at least in its present form.

Point-by-Point Response to the Comments
Reviewer #1: This manuscript reports that Cu metal nanoparticles (NPs) on Al2O3 undergo assisted oxidative dispersion into isolated Cu(II) species under H2O/O2 at room temperature through formation of CuO and then Cu hydroxide.The catalytic activity of the materials with and without the Cu dispersion is compared for CO oxidation and NH3-SCR.
From viewpoints of heterogeneous catalysis and nanomaterial chemistry, the present work is not among the top level considering the following reasons.

Response:
We really appreciate the referee for carefully reading the manuscript and for making insightful suggestions to help us significantly improve the quality of our manuscript.We have performed additional experiments including catalytic activity tests (reverse water gas shift (RWGS) and preferential oxidation of carbon monoxide (CO-PROX) reactions) and more quasi in-situ characterizations to strengthen the catalytic impact and to reveal the underlying mechanisms.Based on the experiment results, we revised our manuscript carefully, and sincerely hope that our revisions have satisfactorily addressed the reviewer's concerns.A new version is attached for review.The following is the point-by-point response: 1.It is well known that Cu/Al2O3 is not effective catalyst for CO oxidation and NH3-SCR.
So, the catalytic impact of this report is low.
Besides high catalytic activity, the regeneration of a sintered catalyst is also crucial, drawing a lot of attentions for decades.It should be noted that the deactivated Cu/Al2O3 and Cu/CeO2 catalysts during high temperature reactions (RWGS and CO-PROX) can be easily reactivated by exposing to O2-H2O atmosphere at RT (Fig. R1).Compared to other regeneration strategy which commonly requires high temperature and specific gases (such as O2, NH3 and CH3I) (Science, 2016, 353, 150-154;J. Am. Chem. Soc., 2019, 141, 4505−4509;ACS Catal., 2012, 2, 552−560), such a simple regeneration method under the ambient condition is environmentally friendly and energy saving.

Response:
We greatly appreciate the reviewer's valuable suggestion.We agree with the referee that we should focus on the formation pathway of hydroxylated Cu-OH species.
We have conducted quasi in-situ EXAFS experiments of Cu NPs under different atmospheres.It is difficult to distinguish the highly dispersed CuOx and Cu(OH)x species due to the same Cu-O bond distance according to the standard spectra of CuO, Cu2O and Cu(OH)2 samples shown in Fig. S5 of the revised SI.Thus, quasi in-situ XPS was used to identify the evolution of Cu(0) or CuO species treated in O2, H2O and O2-H2O.As shown in Fig. R2, no Cu-OH species can be found for Cu NPs in O2 and only a small proportion of Cu-OH species are observed in Ar-H2O.In contrast, a large proportion of Cu-OH species are detected in O2-H2O.If the Cu NPs are firstly oxidized into CuO in O2 and then exposed to Ar-H2O, the proportion of Cu-OH species (10.4%) is much less than Cu NPs in O2-H2O (21.7%) but more than Cu NPs directly exposed to Ar-H2O (7.1%).The result suggests that spontaneous redispersion of Cu NPs in O2-H2O may occur through the oxidation of Cu atoms into atomic Cu-O species followed by the hydroxylation of Cu-O species into Cu-OH species, which migrate and get captured by the support.A new cycle is then repeated until the redispersion process is completed."Quasi in-situ XPS experiments were conducted to identify the surface Cu species after treatment in various atmospheres.Cu 2p3/2 peak located at 932.4 eV is observed in the fresh 2Cu/AlOOH-900 sample (Fig. 2c), which is assigned to Cu + /Cu 0 species 42,43 .The kinetic energy of the main Cu L3VV Auger peak at 916.6 eV and a weak peak around 922 eV indicate that Cu + and a small amount of Cu 0 species coexist on the surface of fresh 2Cu/AlOOH-900 sample 43 (Fig. S6).……" What's more, controlled experiments on the effect of Cu precursors (Cu, CuO and Cu(OH)2 with similar particle size) are conducted.As shown in Fig. R3a, much more highly dispersed Cu species are observed (stronger EPR signal) using Cu(OH)2 as precursor in Ar-H2O.More obvious redispersion can be found for Cu(OH)2 precursor in liquid-phase H2O (Fig. R3b) than gas-phase H2O (Ar-H2O) (Fig. R3a), indicating that increasing H2O amount can significantly promote the redispersion of Cu species.Figs.R3c-f show that bulk Cu(OH)2 can be redispersed into Cu clusters and single atoms on Al2O3 surface in liquid-phase H2O.The above results further demonstrate that the formation of mobile Cu(OH)2 accounts for the rapid redispersion of Cu species (Fig. R3g).(g) Scheme of the effect of migration species on the dispersion of Cu NPs.This is used as Fig. 3 in the revised manuscript.
The above results have been added as Fig. 3 and related discussions are presented on Pages 9 -11 in the revised manuscript: "Cu-OH species dominated the redispersion process.Commercial Cu, CuO, and copper hydroxide (Cu(OH)2) powders with similar particle size have been mixed with AlOOH-900, and then treated in Ar-H2O at RT for 24 h to investigate the effect of Cu precursors on the redispersion process.……." 3. In-situ time resolved EXAFS data is not shown to support the in-situ UV-vis results in Fig. 2a.

Response:
We thank the referee for the helpful advice.Quasi in-situ EXAFS spectra with different treatment times are shown in Fig. R4.The disappearance of Cu-Cu bond (peak at ~ 2.2 Å) and formation of Cu-O bond (peak at ~ 1.5 Å) is much faster in O2-H2O (complete redispersion at 4 h) than the cases in O2 and Ar-H2O (incomplete redispersion at 4 h and even 24 h), which is consistent with the UV-Vis results.The EXAFS results have been added as Fig. 2b and detailed discussions are given on Pages 7, 8 in the revised manuscript: "Subsequently, quasi in-situ XAS experiments were conducted to identify the chemical state of Cu in O2, Ar-H2O and O2-H2O atmospheres.……" Fig. R4 Quasi in-situ Fourier-transforms of k 3 -weighted Cu K-edge EXAFS spectra of 2Cu/AlOOH-900 treated in O2, Ar-H2O and O2-H2O for 4 h and 8 h, as well as standard samples of Cu foil and CuO.This is used as Fig. 2b in the revised manuscript.
Reviewer #2: This work reports an efficient strategy to disperse Cu particles into single atoms or ultrasmall clusters under water-assisted oxidation treatment.Through investigating such a method on various substrates including γ-Al2O3, ZrO2, TiO2, MgO, h-BN, Si3N4, and flake graphite, and combining a variety of different characterizations they concluded that the critical role of H2O is promoting the formation of mobile hydroxylated Cu species and simultaneously providing enriched anchoring sites for the single atomic Cu species.Moreover, the profit of such a particle-size tunability was clearly demonstrated in two model reactions.I consider this is an important work that can provoke the broad interests in the heterogeneous community.The paper has been well organized and well written.Therefore, I would be happy to recommend its publishing on the journal of Nature Communications.A minor revision may be needed according to the following concerns: Response: We appreciate very much the referee's positive comments and valuable suggestions on our work.We have carefully considered all suggestions and performed additional experiments.A new version is attached for the further review.The following is the point-by-point response: 1.There seems to be a clerical error in line 120, Fig. 1e   Interface Anal., 2017Anal., , 49, 1325Anal., -1334) ) indicate that multiple Cu species exist.According to the position of Cu L3VV Auger peak and deconvolution of Cu 2p peaks (Figs.R2, 6), the Cu species in these samples can be exactly recognized: (1) Cu + and Cu 2+ in O2 as proved by the Cu 2p3/2 peaks at 932.4 and 933.4 eV and Cu L3VV Auger peak at 916.4 and 918.1 eV; (2) Cu + , Cu 2+ and Cu-OH in Ar-H2O and O2-H2O as confirmed by the Cu 2p3/2 peaks at 932.4, 933.4 eV and 935.6 eV with corresponding Cu L3VV Auger peak at 916.4, 918.1 and 914.4 eV, respectively.We exclude the existence of Cu 0 species in Ar-H2O and O2-H2O treated samples based on the AES results (Figs.R5, 6).
The Cu 2p and Cu L3VV spectra are updated as Fig. 2c and Fig. S6 in the revised manuscript and SI with corresponding discussions on Pages 8, 9 in the revised manuscript: "Quasi in-situ XPS experiments were conducted to identify the surface Cu species after treatment in various atmospheres.Cu 2p3/2 peak located at 932.4 eV is observed in the fresh 2Cu/AlOOH-900 sample (Fig. 2c), which is assigned to Cu + /Cu 0 species 42,43 .The kinetic energy of the main Cu L3VV Auger peak at 916.6 eV and a weak peak around 922 eV indicate that Cu + and a small amount of Cu 0 coexist in the surface of fresh 2Cu/AlOOH-900 sample 43 (Fig. S6).……" 3.In line 255-256, the conclusion of "The Cu particle size may be the decisive factor for the different catalytic performance in the two reactions" may be a bit overstated.This is because in this specific study, the oxidation state of the Cu species is closely related to the particle size.However, this may not always be true for other recipe of catalyst preparations.
In other words, the valence state of the Cu species may also contribute important roles in these reactions.
Response: Thank the referee so much for the constructive suggestion.We fully agree with the referee that both the valence state and the size of Cu species play important roles in catalytic reactions.In our studies, the enhanced activity is caused by the redispersion of Cu NPs into highly dispersed Cu 2+ species, but we cannot tell whether the size or valence state dominates since the valence state of the Cu species is closely related to the particle size in our study as the referee said.We have conducted more experiments for evaluating catalytic performance (details can be seen in reply to the first referee) and modified the conclusion: "The redispersion of Cu NPs into highly dispersed Cu 2+ species in O2-H2O contributes to the enhanced performance and recovered activity for RWGS and CO-PROX reactions…..." on Page 18 in the revised manuscript.
Reviewer #3: Fan et al. report on the effect of wet pretreatments of supported Cu nanoparticles on metal dispersion.While some interesting data are produced, I am not sure that the article meets the standards of Nature Communications.

Response:
We really appreciate the reviewer for carefully reading the manuscript.After considering these suggestions, we have performed additional experiments and revised our manuscript carefully.We hope that the revision can satisfactorily address the reviewer's concerns.The following is the point-by-point response: 1.The introduction does not summarize the current knowledge on metal redispersion.It is more focused on a few examples dealing with Cu.
Response: Thank the referee very much for the valuable suggestion.We agree with the authors that the current knowledge and understanding of metal redispersion is highly needed in the introduction.Inspired by the referee, we summarize the current knowledge and commonly used methods for metal redispersion.Based on the discussions, we conclude that high temperature and specific atmosphere are usually required for the redispersion but it costs a lot.Our work provides an effective and energy-saving way to achieve redispersion of sintered metal catalysts.We have added the related discussions and updated the abstract and conclusion parts on Pages 1 -3, and 19 in the revised manuscript: "Supported metal nanocatalysts have been widely used in heterogeneous catalysis, while sintering of supported metal species is inevitable during high-temperature reactions leading to catalyst deactivation 1-5 .Numerous redispersion strategies have been developed to reverse the sintering process and rejuvenate the active metal species, which are critical for chemical industries 6-11 ……" 2. The main oxide supports, referred to as AlOOH-500 and -900, are claimed to be γ-Al2O3 (page 4), but there is no evidence for that phase.
Response: Thank the referee for the suggestion.XRD patterns (Fig. R7) confirm the γ-Al2O3 phase in AlOOH-500 and AlOOH-900, which have been added as Fig. S1 in the revised SI.
3. Electron microscopy images in Figs. 1 and 4 show scale bars with no indication of the scale.In Fig. 1a, the supposed single Cu atoms are surrounded by circles that make any visualization impossible.
Response: Thank the referee for pointing out our errors.We have changed the marker from circles to arrows to make visualization clear.The modified Fig. 1 is shown in the revised manuscript.
4. In Fig. 1f, EXAFS data are reported.However, neither XANES curves nor EXAFS fitting is provided.The components at R=4-5 Å are not ascribed for the Cu foil.
Response: Thank the referee for the constructive suggestion.XANES curves and fitting parameters of EXAFS are given in Fig. S4 and Table S1.Our experimental results are consistent with the results in the literature, where the metallic Cu or Cu foil exhibits a bimodal peak at R = 4 -5 Å. (Nature, 2023, 14, 262-269;Nat Commun., 2020, 11, 3525) Table S1.EXAFS distances and fitting parameters for the Cu-based catalysts.Fitting parameters: S0 2 = 0.72 calculated using a Cu foil standard; k and R fit ranges are tabulated.

Fig. 6
with corresponding discussions on Pages 16 -18 in the revised manuscript: "Effect of Cu redispersion on catalytic performance of RWGS and CO-PROX reactions.Due to the high CO selectivity and activity of copper, as well as its low cost compared to gold and platinum, copper-based catalysts may be one of the most promising candidates for RWGS reaction 50,51 ...…." 2. Oxidation of small Cu metal NPs to CuO NPs by air at room temperature is a common sense.Auto dispersion of CuO into atomic Cu(II) via Cu(OH)2 species is a new finding in this work.So, the in-situ characterization should be focused on the pathway of CuO+H2O to Cu(OH)2 to Cu(II) reaction.For example, EXAFS results should include the data for Cu(0)+O2, Cu(0)+H2O, and CuO+H2O.

Fig
Fig. R2 Quasi in-situ Cu 2p XPS spectra of 2Cu/AlOOH-900 after treatment in different atmospheres.The three images on the left are used as Fig.2c in the revised manuscript.

Fig. R3
Fig. R3 Cu precursors effect on dispersion of Cu NPs at RT. (a) Quasi in-situ EPR spectra of physical mixtures of AlOOH-900 and different Cu precursors in Ar-H2O atmospheres for 24 h.(b) EPR spectra of Cu(OH)2-AlOOH-900 before and after water immersion for 24 h.EDX mapping images over Cu(OH)2-AlOOH-900 (c) before and (d) after water immersion for 24 h.(e, f) HADDF-STEM images of Cu(OH)2-AlOOH-900 after water immersion for 24 h.Scale bars are 100 nm and 5 nm in (e) and (f), respectively.
should be Fig 1f.Also, there were no claims of the corresponding lengths for each scale bar in the HAADF-STEM images as shown in Fig. 1.Response: Thank the referee very much for pointing out the errors in our manuscript.We have corrected the clerical error and added the missing claims for scale bar in HAADF-STEM images.Corresponding modifications have been made in the revised manuscript.2.In line 132-135, the XPS observed the formation of Cu 2+ but cannot discern the Cu + and Cu 0 on the O2 and Ar-H2O treated samples.Then the AES was further applied to exclude the existence no Cu 0 species.Could the authors please give a more detailed explanation?Why no Cu 2+ species was recognized on the AES spectra?Response: Thank the referee so much for the professional suggestions.To monitor the evolution of Cu species in different atmospheres, quasi in-situ XPS tests with different treatment times are re-conducted.Commonly, Cu 2p3/2 peak can be used to identify Cu 2+ (BE: ~ 933.6 eV ± 0.2 eV) and Cu + /Cu 0 (BE: ~ 932.4 eV ± 0.2 eV), and Cu L3VV Auger peak is applied to distinguish Cu + (KE: ~ 916.4 eV ± 0.2 eV) and Cu 0 (KE: ~ 918.6 eV ± 0.2 eV), of which the latter is close to Cu 2+ (~ 918.1 eV ± 0.2 eV).

Fig
Fig. R5 Cu 2p XPS spectra and Cu L3VV Auger spectra of Cu, Cu2O and CuO.(From

a
Atomic distance; b Coordination number; c Difference of potential between the sample and the standard; d Debye-Waller factor

Response:
Thanks a lot for the important suggestions.Quasi in-situ EXAFS results of the fresh 2Cu/AlOOH-900 and samples treated with O2/Ar-H2O/O2-H2O for different times are shown in Fig.R4, which confirm the rapid redispersion into single atoms in O2-H2O, of which details can be seen in the reply to the first referee.The EXAFS results have been added as Fig.2band detailed discussions are given on Pages 7, 8 in the revised manuscript: "Subsequently, quasi in-situ XAS experiments were conducted to identify the chemical state of Cu in O2, Ar-H2O and O2-H2O atmospheres.……" 6.The XPS data in Fig.2bare hardly visible, hidden by thick fitting curves.Moreover, why are the 2p1/2 components not shown?Response: Thank the referee for the nice advice.We have decreased the thickness of the fitting curves to make them clear.Cu 2p1/2 components are added (Fig.R2).Fig.2cis updated in the revised manuscript accordingly.7. H-D exchange experiments: describe them and how they can be interpreted in terms of OH content, since nothing can be understood just from the claim "H-D exchange results show that Ar-H2O treatment increases the OH content…".Response: Thank the referee for the nice suggestion.For H-D exchange experiment, the pretreated sample (Ar, 200 °C for 2 h) was exposed to D2 with temperature increased from RT to 750 ℃, and the change in the mass spectroscopy (MS) signals of HD (m/z = 3) was

Table R1 .
EXAFS distances and fitting parameters for the Cu-based catalysts.Fitting parameters: S0 2 = 0.88 calculated using a Cu foil standard; k range: 2.3-11 Å -1 . 5.Moreover, still for EXAFS, why only 2Cu/AlOOH-900 sample treated in O2-H2O is reported?One would at least expect the data for this sample before treatment.